Flat multiconductor cable for undercarpet wiring system

ABSTRACT

The flat multiconductor cable contains live, neutral and ground conductors lying generally parallel to each other and in side by side, single-plane relation. The live and neutral conductors are sealably surrounded by a relatively hard, polyetherimide material that is substantially free from attack from calcium hydroxide. An outer jacket or casing of polyvinylchloride (PVC) sealably surrounds the polyetherimide layers and contacts the ground conductor. The PVC is substantially free from attack in an alkaline environment. As such, the cable construction provides an enhanced resistance to puncture, particularly in alkaline environments which may be encountered in undercarpet wiring system applications.

FIELD OF THE INVENTION

The present invention relates generally to an electrical cable and moreparticularly to a flat multiconductor cable for use in an undercarpetwiring system.

BACKGROUND OF THE INVENTION

Undercarpet wiring systems for power, data and telephone applicationshave been in commercial use for several years. One such wiring system ismanufactured by the assignee of the subject invention and is sold underthe trademark VERSA-TRAK. The wiring system for power applications, forexample, typically includes a flat cable comprising three, four or fiveconductors encased in a jacket or casing of insulation. A conductiveshield overlies the cable for mechanical and electrical protection andis electrically connected to the ground conductor such that the shieldand ground conductor are at the same ground potential. In theVERSA-TRAK® system, the ground shield is connected intermittently alongthe cable length to the ground conductor by welds. An insulative film isoften attached to the bottom of the casing for further cushioning andabrasion shielding. U.S. Pat. Nos. 4,219,928 and 4,283,593, both ofwhich are assigned to the assignee of the present invention, describe aknown electrical cable asssembly for use in undercarpet wiring systems.

In use, the cable assembly is adapted to be placed on a floor surface incommercial buildings and thereafter covered with carpet squares. As thecable with shielding is relatively thin (generally less than 1/16 inch),the wiring system remains unobtrusive once installed. It has been foundthrough experience and tests that certain elements in the environment towhich the undercarpet wiring system is exposed could present problems inuse. These elements include dirt particles or other debris and water.During installation or use, particles of dirt may get lodged between thecable casing and the metallic shield or between the cable and the floorthereby increasing the potential for puncturing the cable and eitherdamaging the system or shutting down the system by tripping a circuitbreaker through a short circuit. In addition, and in particular wherethe undercarpet cable is placed on a concrete floor on the grade levelof a building, water may be vaporized from the concrete surface and beretained under the carpet after condensing. The water may combine withthe chemicals in the concrete to form an alkaline solution consistingmainly of calcium hydroxide. Calcium hydroxide and other alkalinesolutions are known to attack certain materials, such as polyesters,which are commonly used in the undercarpet cable insulation because theyhave good dieletric and hardness properties. It is known, for example asdisclosed in the above-referenced patents, to form a composite jacket ofinsulation with a harder layer of polyester, such as MYLAR film (MYLARis a trademark of E. I. Du Pont de Nemours) on the jacket exterior witha softer layer of polyvinylchloride on the interior of the jacket. It isalso known that undercarpet cables use one or two layers of MYLAR filmas the insulation jacket, the film being laminated to the conductors.

Steps have been taken in the industry to ensure safe and reliableundercarpet systems. For example, as a means of providing a moisturebarrier, especially on grade level concrete floors, a separate strip ofpolyvinylchloride (PVC) is laid on the floor beneath the cable (orbeneath the abrasion shield). This PVC strip is several inches widerthan the width of the cable and serves as a moisture barrier to water orcalcium hydroxide when present on the floor. This PVC strip furtherserves as an additional cushion for minimizing puncturing of the cablefrom dirt particles between the cable and the floor.

Other attempts not presently adapted commercially include efforts atsealing the edges of the cable assembly so that water, dirt and otherdebris may be kept outside the cable assembly. One such approach isshown in U.S. Pat. Nos. 4,417,096 and 4,319,075, both issued to Willetteand assigned to AMP, Incorporated. In addition to costs, obtainingentrance to the sealed cable assembly for making taps or splices, andthen re-sealing the cable assembly present problems.

While current undercarpet systems have been improved for safety andreliability, there is a continued need for providing solutions to thewater and debris concerns in a cost effective manner.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedelectrical cable for use in an undercarpet wiring system.

It is another object of the invention to provide an electrical cable foruse in an undercarpet wiring system, the cable being substantially freefrom attack by alkaline solutions.

In accordance with a preferred form of the present invention, anelectrical cable is provided for use in an undercarpet wiring system,the cable being likely to be exposed to an alkaline environment. Thecable comprises a plurality of elongate electical conductors extendinggenerally parallel to each other and in side-by-side relation. Theconductors include live, ground and neutral conductors. A layer ofinsulation individually surrounds each of the live and neutralconductors. A casing of insulation surrounds the conductors and is inadhered relation to the respective layers of insulation surrounding thelive and neutral conductors. The casing substantially surrounds theground conductor and is in contact therewith. The casing of insulationis substantially free from attack in an alkaline environment.Preferably, the layer of insulation is also substantially free fromattack in an alkaline solution containing calcium hydroxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a cable assembly constructed inaccordance with one aspect of the present invention.

FIG. 2 is a cross-sectional view as seen along viewing lines II--II ofFIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing in FIG. 1, there is shown an electricalcable assembly 10 including a flat, flexible elongate multiconductorcable 12 covered by a self-sustaining electrically conductive memberconstituting a metallic ground shield 14. The cable assembly 10 isadapted to be installed on a floor surface such as a concrete surface 16in a commercial building and to be covered by carpet squares 18 or othersimilar types of floor covering. As such, cable assembly 10 may be usedin an undercarpet wiring system which is becoming more popular inbuilding wiring applications.

With reference still to FIG. 1 and also now to FIG. 2, the details ofthe cable assembly may be described. The multiconductor cable 12contains a plurality of elongate electrical conductors 20, 22, and 24,extending generally parallel to each other and in side by side,single-plane relation. In the cable illustrated, conductor 20 serves asa live conductor, which for example, may be connected to A, B or C phasefor load balancing purposes. Conductor 22 located adjacent to the liveconductor, serves as a ground, and conductor 24 serves as a neutralconductor. It should also be understood that other multiconductor cableshaving more than three conductors such as four conductors (where theremay be two live conductors) or five conductors (where there may be threelive conductors) are also contemplated in the present invention. In thepreferred embodiment, conductors 20, 22 and 24 are of generallyrectangular cross-section having a thickness of about 0.006 inch andformed of copper.

The metallic shield 14 is made from a thin flexible sheet of suitablyelectrically conductive metal, preferably copper, substantially the samematerial as the conductors 20, 22 and 24. The shield 14 is electricallyand mechanically secured to the ground conductor 22 at intermittentlocations along the length of the cable 12. The intermittent spacingsare approximately twenty-seven (27) inches in the preferred arrangementalthough other suitably spaced distances may be used. In the preferredcable construction, securement of the ground shield 14 and groundconductor is effected by a weld 26 as more particularly shown anddescribed in aforementioned, commonly-owned U.S. Pat. No. 4,283,593. Itshould be understood as set forth in the '593 patent that othersecurement means, such as mechanical fasteners and rivets or the likemay be used in securing the shield 14 and the ground conductor 22 suchthat they would be at the same electrical potential. Between securedlocations (welds 26) the shield 14 is not secured to the cable 12 and isreadily separable therefrom so as to facilitate taps or splices to othercable assemblies.

The metallic shield is preferably made on the order of 0.006 inch thickand has a width at least sufficient to cover the live and neutralconductors and extend beyond the longitudinal edges of the cable 12. Assuch, the wider shield provides a "feathering" effect in minimizing theperceived height of the cable assembly once installed, the shield edgesbeing attached to the floor surface by a suitable adhesive tape 28 orthe like. The shield may be suitably plated with nickel or the like toincrease corrosion resistance. The underside of the shield 14 maycontain an insulative sheet 30 disposed on either side of the groundconductor 22. Sheets 30 may be made of polyvinylchloride, about 0.004inch thick and laminated to the shield 14. These sheets 30 act ascushions to any debris or dirt particles that may lodge between theshield 14 and the cable 12 and thereby help to minimize puncture of thecable.

In the preferred form of the invention, the live conductor 20 and theneutral conductor 24 are completely, individually surrounded with alayer 32, 34 of relatively hard, insulative material having gooddielectric strength. The ground conductor 22 remains free from such alayer of insulation. An outer casing 36 of insulative material, also ofgood dielectric strength, surrounds the insulative layers 32, 34 and theground conductor 22. In accordance with the invention, the cable isadapted to be substantially resistant to attack in an alkalineenvironment. As noted hereinabove, an undercarpet cable is likely toencounter an environment containing alkaline solutions, particularlycalcium hydroxide which may form as a result of water combining withcalcium in concrete floors, mainly at grade levels in buildings.Accordingly, the outer jacket 36 is selected to be substantially freefrom attack in such alkaline environments. As used herein, the phrase"substantially free from attack" is intended to mean that the materialwill not crack, decompose or weaken in its normal use. In the preferredconstruction, polyvinylchloride (PVC) is selected for its ability to besubstantially free from attack in alkaline solutions.

In addition to alkali resistance capability, the cable of the presentinvention is adapted to provide improved puncture resistance over theknown commercially available cables, especially in an alkalineenvironment. In the particularly preferred form of the cableconstruction, the material of the insulative layers 32, 34 is apolyetherimide, such as that sold under the trademark ULTEM andavailable from General Electric Company, One Plastics Avenue,Pittsfield, Mass. ULTEM material is also preferred as it has excellentresistance against attack from alkaline slutions containing calciumhydroxide. The ULTEM layers 32, 34 are provided on the conductors at athickness of about 0.005 inch. The casing thickness around the layers32, 34 is about 0.007 inch and the overall casing thickness surroundingthe conductors is about 0.030 inch. In the present cable configuration,the outer casing of PVC is not attacked or otherwise weakened byalkaline solutions, maintaining thereby the puncture resistance of thecable in its intended environment. On the other hand, in the known cablestructures, an exterior polyester film weakens when exposed to alkalinesolutions, thereby permitting the cable insulating to be more easilypunctured under external forces of a particle or the like.

In assembly of the cable assembly, the ULTEM layers 32, 34 arepreferably extruded onto the conductors 20 and 24 by known extrusiontechniques. The extruded layers adhere to the copper conductors,eliminating thereby the need for separate adhesives and generallyproviding a seal between the layers 32, 34 and the conductors 20, 24 andthereby a moisture barrier. The casing 36 is also then monolithicallyplaced by extrusion over the layers 32, 34 and over the ground conductor22. Goods adherence and thereby a good seal is also obtained between thecasing 36 and the layers 32 and 34. As there is no ULTEM material aroundthe ground conductor 22, portions of the relatively soft, flexiblepolyvinylchloride at the weld locations are readily removable withoutinterrupting the moisture seals around the live conductor 20 and neutralconductor 24 to facilitate welding of the ground shield 14 to the groundconductor 22 at the intermittent spacings. The casing 36 may be providedwith score marks 40 located between the conductors 20, 22 and 24 on bothsides of the casing 36, creating thinned casing sections so as tofacilitate separation of the cable. Splitting the cable along the scoremarks 40 will enable separation of the cable into sections, eachcontaining a conductor for subsequent termination or connectionpurposes.

It can now be appreciated that the cable assembly of the presentinvention provides an improvement for undercarpet wiring systems. Whilethe preferred embodiments of the cable have been described herein, itshould be understood that other variations may be made thereto withoutdeparting from the contemplated scope of the invention. As such, thepreferred embodiments described herein are intended in an illustrativeand not a limiting sense. The true scope of the invention is set forthin the claims appended hereto.

We claim:
 1. An electrical cable for use in an undercarpet wiring systemwherein said cable is likely to be exposed to an alkaline environment,said cable comprising:a plurality of elongate electrical conductorsextending generally parallel to each other in side by side relation,said conductors including live, ground and neutral conductors, a layerof insulation individually surrounding each of said live and saidneutral conductors, a casing of insulation surrounding said conductorsand being in adhered relation to said respective layers of insulationsurrounding said live and said neutral conductors, said casingsubstantially surrounding said ground conductor and being in contacttherewith, said casing of insulation being substantially free fromattack in an alkaline environment.
 2. An electrical cable according toclaim 1, wherein said casing material comprises polyvinylchloride.
 3. Anelectrical cable according to claim 1, wherein said alkaline environmentcomprises calcium hydroxide and wherein said layer of insulation issubstantially free from attack from said calcium hydroxide.
 4. Anelectrical cable according to claim 3, wherein the material of saidlayer of insulation comprises polyetherimide.
 5. An electrical cableaccording to claim 4, wherein said material of said layer of insulationconsists essentially of polyetherimide.
 6. An electrical cable accordingto claim 1, wherein said casing comprises a material softer than thematerial of said layers of insulation surrounding said live and neutralconductors.
 7. An electrical cable according to claim 1, wherein saidlayers of insulation surrounding said live and said neutral conductorsare adhered thereto.
 8. An electrical cable according to claim 1,wherein said layers of insulation surrounding said live and said neutralconductors are extruded thereover.
 9. An electrical cable according toclaim 8, wherein said casing is extruded over said layers of insulationand over said ground conductor.
 10. An electrical cable according toclaim 1, further including an electrically conductive shield overlyingsaid casing and extending longitudinally therealong, said shield havinglateral extent to extend over at least said live and said neutralconductors and being electrically connected to said ground conductor.11. An electrical cable according to claim 1, wherein said casingincludes means located between said conductors for facilitating divisionof said cable into separate portions each containing a conductor.
 12. Anelectrical cable for use in an undercarpet wiring system comprising alive conductor, a ground conductor and a neutral conductor extending ingenerally parallel, side by side, spaced relation, a layer of relativelyhard insulation individually adheringly surrounding each of said liveand neutral conductors, and a casing of insulation surrounding saidconductors, said casing being in adhered relation to said respectivelayers of insulation and substantially surrounding and contacting saidground conductor and arranged thereon such that removal of a portion ofsaid casing will expose said ground conductor for making electricalconnection thereto.
 13. An electrical cable assembly according to claim12, wherein said cable casing comprises a material that is substantiallyfree from attack in an alkaline environment.
 14. An electrical cableassembly according to claim 12, wherein the material of said casing andthe material of said layers of insulation are substantially free fromattack in an alkaline environment containing calcium hydroxide.
 15. Anelectrical cable according to claim 12 further including an electricallyconductive shield extending lengthwise with said conductors andoverlying said casing, said shield having a lateral extent tosuperimposingly cover all said conductors, said shield being secured atintermittent lengthwise locations to said ground conductor whereportions of said casing have been removed and being unsecured to saidcasing between such secured locations.
 16. An electrical cable assemblyaccording to claim 15, further including a sheet of insulation disposedbetween said shield and said casing.
 17. An electrical cable assemblyaccording to claim 15, wherein said shield includes means disposedthereon to minimize corrosion.
 18. An electrical cable assemblyaccording to claim 15, wherein said shield is welded to said groundconductor at said secured locations.
 19. An electrical cable assemblyaccording to claim 15, wherein said ground conductor and said shield aremade of substantially the same material.